The present invention relates to methods of inkjet printing.
There are a number of well-known inkjet printing techniques in which the number of ink droplets deposited per unit area is controlled to vary the optical density of printing. With multiple printheads, print colour can similarly be controlled.
A problem that is frequently encountered is that the final print quality is heavily dependent on whether and to what extent individual ink droplets spread on the substrate and coalesce with their neighbours. The behaviour of any one ink droplet in this respect is dependent to a considerable extent upon the number of other ink droplets being deposited in the same vicinity. Moreover, this behaviour is also influenced by microscopic variations in the mechanical and chemical properties of the substrate (especially the roughness of the substrate surface and the surface tension), so that the degree of spread and coalescing of droplets is not constant or reproducible, even where the pattern of neighbouring droplets is unchanging.
To provide a degree of control over the behaviour of ink droplets on the substrate, and indeed during the droplet deposition process, it is common to use inks that have a defined curing, fixing or hardening phase, such as ultra-violet curable ink. Whilst remaining within the inkjet printhead, during flight and for an initial interval upon the substrate, this liquid UV curable ink remains in what might be termed a xe2x80x9cwet phasexe2x80x9d. At the appropriate time, the printed droplets are exposed to UV radiation, effecting through UV curing a transformation from the xe2x80x9cwet phasexe2x80x9d to a xe2x80x9cdry phasexe2x80x9d. A similar effect can be achieved with hot melt inks where the transformation from xe2x80x9cwet phasexe2x80x9d to xe2x80x9cdry phasexe2x80x9d is controlled by temperature, or with dual-component inks where exposure to a hardening or fixing component effects the transformation. In this specification, the terms xe2x80x9cwetxe2x80x9d and xe2x80x9cdryxe2x80x9d will be employed to denote the respectively less and more viscous states of an ink or other printing material which has a defined curing, fixing or hardening phase.
It is an object of one aspect of the present invention to provide an improved method of inkjet printing in which the variability of droplet behaviour on the substrate is significantly reduced.
Accordingly, the present invention consists, in one aspect, in a method of ink jet printing on a substrate, comprising the steps of forming a wet undercoat layer on the substrate; depositing onto the undercoat layer, whilst the undercoat layer remains wet, a pattern of wet ink droplets and subsequently transforming the undercoat layer and deposited ink droplets to a dry state.
The undercoat layer may be formed using a variety of well known techniques, such as offset or bar coating, although it is preferably formed, wholly or in part, through ink jet printing.
The undercoat will typically be colourless, although this is not essential. A white undercoat may, for example, be useful in concealing colour variations over the substrate. If the undercoat is a different colour than the intended print substrate, there may be a requirement for further image processing of the print data prior to printing.
It is found that the spreading and coalescing of wet ink droplets on a wet undercoat is considerable more uniform and reproducible than on a bare substrate.
In many applications, the described problem of variable behaviour of deposited ink droplets is noticeable at certain densities of printing but not at others. It may be, for example that at low densities, the deposited ink droplets are sufficiently spaced that they never coalesce. At high densities, the ink droplets overlap sufficiently for them always to coalesce. It is over an intermediate range of densities that the problem of variable droplet behaviour produces noticeable artefacts.
Accordingly, in a preferred form of the invention, the step of forming a wet undercoat layer comprises the formation of a layer that varies spatially with the pattern of ink droplets to be deposited.
Suitably, the undercoat layer is formed on selected regions of the substrate but not on other regions, the selection of regions to receive an undercoat being dependent upon the pattern of ink droplets.
Advantageously, the thickness of the undercoat layer varies spatially with the pattern of ink droplets to be deposited.
Whenxe2x80x94for examplexe2x80x94UV printing with 100% reactive inks, a glossy finish is usually produced. Reflected light from the surface reveals variations in thickness across the image.
It is observed that in such and certain other applications, the print quality can be further improved if the thickness of the undercoat layer varies generally inversely with the number of ink droplets to be deposited in a local region. In this way, it can be arranged that the total thickness of undercoat plus ink, remains sensibly constant over the substrate. This step is particularly beneficial where variations of print density are relatively gradual, such as in the printing of photographs or other images. Where there are high spatial frequencies in the print contentxe2x80x94such as with textxe2x80x94and the desired changes in print density are abrupt rather than gradual, the step may be less beneficial orxe2x80x94indeedxe2x80x94harmful.
Thus, in a further form of the invention, the variation in the thickness of the undercoat layer with the pattern of deposited ink droplets, is disabled in regions where the print content to be represented by the pattern of ink is determined to comprise text or other high spatial frequency matter.